In cold weather diesel engines are sometimes associated with starting problems, sluggish diesel fuel, i.e. diesel with high viscosity, and the necessity to use anti-gel additives. In other words, the biggest problem with running diesels in cold weather is the tendency of the fuel to gel. A type of diesel recommended for most passenger vehicles contains some naturally occurring paraffin (wax) and as the temperature drops, this paraffin crystallizes and affects the fluidity of the fuel and may cause hard starting and eventually lead to filter plugging/clogging. Unfortunately, this problem is exacerbated when biodiesel enters the equation—biodiesel tends to gel at a slightly higher temperature than diesel.
Luckily these problems are fairly easily solved. Regular diesel fuel is “winterized” or seasonally adjusted at the distributor before it's delivered to the pumps. Winterizing is done by mixing the type of diesel mentioned above with a more refined diesel. Winterizing diesel fuel is done to maintain the cold weather flow characteristics, and the ratios vary depending upon regional distribution. To effectively use biodiesel in cold climates, it must be mixed with winterized diesel in varying percentages, which, once again, are regionally dependent.
For hybrid vehicles with a diesel engine as the ICE (internal combustion engine) and especially for plug-in hybrids there enters a further problem. When the hybrid vehicle is driven solely by the electric motor the fuel will not even be flowing which might lead to the diesel getting colder. Therefore, to make sure that there is no clogging of the filter the ICE is always run if the ambient air drops below a predetermined temperature, for instance, 0° C. Even though some diesel fuels are “winterized”, to be on the safe side, the ICE will be running and thereby the vehicle will more or less function as a standard vehicle without an electric motor at low temperatures.
A further problem arises with a plug in hybrid vehicle that is possible to run purely in electric mode for a long time, i.e. if for instance someone has a short commuting distance and recharge the battery every night or during the day when at work. In a worst case scenario such a vehicle still have diesel in the tank from the summer season when winter arrives.